Noise has long been regarded as a harmful form of environmental pollution mainly due to its high penetrating power. Current noise shielding solutions are directly tied to the mass of the barrier. In general, noise transmission is governed by the mass density law, which states that the acoustic transmission T through a wall is inversely proportional to the product of wall thickness l, the mass density ρ, and the sound frequency f. Hence doubling the wall thickness will only add (20 log 2=) 6 dB of additional sound transmission loss (STL), and increasing STL from 20 to 40 dB at 100 Hz would require a wall that is eight times the normal thickness.
Although a number of structures have been used to improve the STL, they have a limited effective bandwidth and their performance varies depending on the temperature and external distortions. Many instances require a material with high STL over a large bandwidth and tolerance of high environment variations.
The prior art discloses different approaches to achieving at least partial sound transmission losses. For example, U.S. Pat. No. 7,510,052 discloses a sound cancellation honeycomb based on modified Helmholtz resonance effect. U.S. Application 20080099609 discloses a tunable acoustic absorption system for an aircraft cabin that is tuned by selecting different materials and changing dimensions to achieve soundproofing for each position and specific aircraft. Unfortunately, the structures disclosed in U.S. Application 20080099609 are heavy and bulky. U.S. Pat. No. 7,263,028 discloses embedding a plurality of particles with various characteristic acoustic impedances in a sandwich with other light weight panels to enhance the sound isolation. Although it could be lighter or thinner than traditional solid soundproofing panels, it is still bulky and its soundproofing operating frequency is high which makes it less effective for low-frequency operation. U.S. Pat. No. 7,249,653 discloses acoustic attenuation materials that comprise an outer layer of a stiff material which sandwiches other elastic soft panels with an integrated mass located on the soft panels. By using the mechanical resonance, the panel passively absorbs the incident sound wave to attenuate noise. This invention has a 100 Hz bandwidth centered around 175 Hz and is not easily tailored to various environmental conditions. U.S. Pat. Nos. 4,149,612 and 4,325,461 disclose silators. A silator is an evacuated lentiform (double convex lens shape) with a convex cap of sheet metal. These silators comprise a compliant plate with an enclosed volume wherein the pressure is lower than atmospheric pressure to constitute a vibrating system for reducing noise. To control the operating frequency, the pressure enclosed in the volume coupled with the structural configuration determines the blocking noise frequency. The operating frequency dependence on the pressure in the enclosed volume makes the operating frequency dependent on environment changes such as temperature. U.S. Pat. No. 5,851,626 discloses a vehicle acoustic damping and decoupling system This invention includes a bubble pack which may be filled with various damping liquids and air to enable the acoustic damping. It is a passive damping system dependent on the environment. Finally, U.S. Pat. No. 7,395,898 discloses an antiresonant cellular panel array based on flexible rubbery membranes stretched across a rigid frame. However, the materials disclosed in U.S. Pat. No. 7,395,898 limit the bandwidth to about 200 Hz and a single attenuation frequency.
Embodiments disclosed in the present disclosure overcome the limitations of the prior art and provide improved STL.